Method of assorting, dewatering or the like of fibrous material suspended in water



3,024,911 HE LIKE OF IN WATER INVENTOR.

A TTORNEY.

March 13, 1962 B. G. JANSON METHOD OF ASSORTING. DEWATERING OR T FIBROUS MATERIAL SUSPENDED Filed Sept. 14, 1959 BENG'T GOTTHAR JWSO/V United States Patent Office 3,024,911 Patented Mar. 13, 1952 3,024,911 METHOD OF ASSORTING, DEWATERING OR THE LIKE F FIBROUS MATERIAL SUSPENDED IN WATER Bengt Gotthard Janson, Vallvik, Sweden, assignor to Aktiebolaget Celleco, Stockholm, Sweden, a corporation of Sweden Filed Sept. 14, 1959, Ser. No. 839,585 Claims priority, application Sweden Sept. 15, 1958 1 Claim. (Cl. 209268) An especially efiicient method of assorting and/or dewatering a material suspended in Water resides in spraying the suspension in one or more distributed, diverging or expanding jets e.g. from centrifugal nozzles so as to cause the jets to strike against one side (the front side) of a screen wall, water and smaller material particles then passing through the screen wall while larger particles are caught on the front side of the same. Such a method has been successfully applied, for instance, in removing the resin from resin containing cellulose pulp. Hereby the short resin fibres, water and the free resin are separated through the screen Wall.

The center axis of the jet is suitably directed substantially transversely to the screen wall and the partial jets about said axis strike the screen wall more and more obliquely to the striking or impact surface the longer the radial distance is between the partial jets and the center of the jet. The particularly favourable result is primarily due to the fact that by the expanding shape of jet the partial jets about the center axis of the jet exert, in all directions from the impact surface, a splitting effect and a removing effect on the mat formed on this surface so that the screen surface is self-cleaned at this area.

Principally it seems to be obvious that this as well as other screening methods could be exercised to advantage by means of a vacuum on the back side of the screen wall, but this combination has not been utilized previously which probably is due to the following special circumstances connected with the method.

It has become evident that for a given screen wall the screening effect varies with the surface load, i.e. the material supplied per impact surface unit, according to a curve which has a pronounced maximum at a certain surface load. If the amount of material supplied is changed, thus also the impact surface should be changed which according to a known method may be performed by varying the distance from the nozzle to the screen wall. On account of the shape of the jet, which diverges from the nozzle, an increase or a decrease of this distance will cause an increase or a decrease, respectively, of the impact surface. The impact surface may also be varied by changing the diverging angle of the jet. Thus, if an apparatus intended to operate according to the method is to be suitable for various loads, it must be adapted to enable said variations to be performed, that is, beside the fact that the impact surface must be varied the form and size of the screen wall must be dimensioned for the greatest impact surface aimed at and consequently it must be overdimensioned for all smaller impact surfaces. This means that at least in all those cases where the apparatus is not operated under maximum loads certain parts of the screen wall, which in dependence of the load are larger or smaller, will lie outside the impact surface.

Hence, if it is to be possible to maintain an eflicient vacuum Within the effective screening surface, that is, within the impact surface, without too great a power consumption for the vacuum formation those portions of the screen wall which lie outside the impact surface must be covered, these portions otherwise forming a too unobstructed passage for the air. However, how this is to be brought about is a problem, as the area of these portions varies in size and shape with variations of the impact surface. Such considerations may form a sufficient reason for giving up attempts to utilize the vacuum method.

However, the inventor has found by trials of his own that applying a properly balanced vacuum will cause that material, which is propelled by the jet across the screen wall outside the impact surface, will form a mat against the screen wall around the impact surface with the desired covering effect as a consequence.

On the basis of these trials the invention is characterized in that a vacuum is maintained on the back side of the screen wall, said vacuum being adjusted to cause material from the jets to form a layer of mat on the area about the impact surface, said material being driven out along the screen wall about said surface by means of the jets.

In this way a covering is obtained by the felted or matted layer and said covering adjusts itself automatically in size and shape to the extension of the impact surface. If the impact surface is increased that portion of the covering which momentarily arrives within the enlarged impact surface will be quickly driven off from said surface, due to the self-cleaning effect disclosed above. On the other hand, if the impact surface is reduced that portion of the screen which is momentarily uncovered next to the reduced impact surface will be quickly covered by material driven out from this impact surface.

For a further understanding of the invention reference is made to the accompanying schematic drawing which provides an example of an apparatus for carrying the invention into effect. In that drawing:

FIG. 1 is a vertical section of such apparatus;

FIG. 2 is a similar view but omitting certain specific details of the apparatus according to FIG. 1 and showing the remainder in an alternative working position.

In FIG. 1 a casing 1 encloses a chamber 2 having a bottom outlet 3 and a chamber 4 having a bottom outlet 5, said chambers being separated by a strainer wall 6. A centrifugal spray nozzle 7, preferably centrally located in the chamber 2, receives suspension to be treated through a pipe 8 and a hose 9 from a receptacle for such suspension (not shown) to atomize said suspension and to direct a full conical jet 10 of atomized suspension against the strainer wall 6 with the central axis 11 of that jet directed perpendicularly to said wall 6. All particles of the jet stream about that central axis will thus move along trajectories, for instance, 11a and 11b, which incline more with respect to the central axis the more radially distant from that axis they are situated. In that way, within the impact area 6a all parts of the jet about the center line at the moment of impact will have a motion component parallel to the strainer wall outwards from the center of that area. Thus suspended material that will not pass through the strainer wall is driven out by that component in all radial directions from the center. In this way self cleaning of the impact area is obtained, and the material which otherwise would cause formation of a cake over that area is moved out over the surrounding strainer area 6b, over which it passes out through the outlet 3 without adhering to said surrounding area, at least not in any regular way. Material passing through the strainer wall will escape through the outlet 5.

By loosening the set screw 12 the nozzle 7 may be set at different distances from the strainer wall. FIG. 2 illustrates a setting in which the distance 16 is shorter than according to FIG. 1. In consequence thereof, the impact area 6a is less and the surrounding area 6b is larger than in FIG. 1. The object of this adjustability is to enable variation to be made of the specific load of the effective strainer area.

By means of a pipe 13 with valve 14 which communicates with an evacuation device (not shown) a vacuum may be maintained in the chamber 4 on the opposite side of the strainer wall 6 from the chamber 2. This vacuum is regulated according to the invention so that it is great enough to cause material moved out from the impact area over the surrounding area to form a cake 15. It is not so great, however, as to cause this cake to adhere so strongly to the strainer surface that as the nozzle is moved, for instance, from a position according to FIG. 2 to a position according to FIG. 1, whereby the impact area is enlarged, the power of the jet becomes insufficient to drive parts of that cake that otherwise would remain within the enlarged impact area out of that area.

The most suitable degree of vacuum may vary from case to case in dependence of e.g. the character of the suspension and the screen wall, and presumably it may only be established empirically. However, when one once has realized that the way stated is feasible it is a comparatively simple measure to make the necessary experiments. One should then not only see to it that the covering aimed at is obtained but also that it is not so firmly sucked on that the available jet power is unable to remove the same from the impact surface when this is enlarged. It has been shown that a vacuum of about meters of water column would normally be suitable. In most cases a suitable vacuum may thus be provided by such a simple means as a barometric down-pipe from an enclosed chamber behind the screen wall, said pipe being used as an outlet for the passing-through portion of the suspension. Of course the vacuum may also be generated by means of some other known suction device.

The following comparison may serve as an example of the efiect obtained by the invention.

A cellulose pulp having a dry content of about 1% was thrown through a centrifugal nozzle in the shape of a conical jet in a horizontal direction transversely against a vertical screen cloth having a mesh size of 100p and a free passage area of about 32%.

The surface area of the screen cloth was about 18 dm. within which an impact surface of the jet of about 12 dm. was obtained.

At a specific load of about 32 kgs. dry substance per m? effective screen area per minute, a thickening of the pulp to about 1.6% dry substance content was obtained Without vacuum, at which concentration the pulp fiowed off from the front side of the screen cloth without forming any remaining layer against the same.

Under circumstances unchanged in other respects a vacuum of about 5.5 meters of water column was then created on the back side of the screen cloth. Hereby a covering layer of fibres was formed over the whole screening surface outside the impact surface, and the thickened pulp flew across said layer in a concentrated state of about 2.3% dry substance content.

What I claim is:

In the method for asserting, dewatering and the like of material suspended in water by atomizing and impinging such suspension in the form of a full conically diverging jet against one side of a screen wall with the centerline of the jet directed substantially transversely to the screen wall by means of a centrifugal spray nozzle the distance of which from said screen wall may be varied so as to cause the impact area of the jet against the screen wall to cover a larger or smaller part of the screen wall, the steps of causing said jet to build up a layer of suspended material that does not penetrate said screen wall on said one side of said screen wall about said irnpact area, applying a vacuum to the other side of said screen wall, adjusting said vacuum to allow said suspended material to be continuously driven out from said impact area over the screen surface in all directions about the central part of said impact area by the splitting action of said jet and to form a cake of said layer of said out driven suspended material over the remaining part of said screen wall about said impact area.

References (lited in the file of this patent UNITED STATES PATENTS 2,636,612 Cording Apr. 28, 1953 2,799,394 Boogaard July 16, 1957 FOREIGN PATENTS 560,144 Italy Apr. 1, 1957 

